Zero-crossing point detection circuits are used for detecting instances at which a signal changes from a negative to a positive polarity and from a positive to a negative polarity. For example, in an alternating voltage signal (e.g., a sinusoidal source), a zero crossing is an instantaneous point at which there is no voltage present, such as when the signal transitions through zero. This occurs twice during each cycle.
Zero crossing detection circuits may be used in many instances, such as to detect a zero cross in solid state relays to turn on the solid state relay as close to the zero crossing as possible. Zero crossing detectors are also used in systems to coordinate operation. Devices plugged into AC power can keep track of the zero crossing to perform various timing dependant operations as each device sees the same AC power, for example.
As another example, an electronic control circuit for automatic transfer switches and pump controllers may require accurate zero crossing detection of AC power sources for computing root-mean-square (RMS) voltages, frequency, and in phase transfers. For applications with in phase transfers, the zero crossing signals are used to determine the appropriate time to initiate a transfer. As still other examples, the zero crossings for multiple phases in a three phase system can be used to determine phase rotation of the sources.
Besides the inconvenience and annoyance of false zero crossing signals, serious conditions can arise when false zero crossings occur. For example, RMS voltages can become inaccurate, switches can inappropriately transfer, phase rotation errors can result, and in some cases, transfers can be inhibited. False zero crossings for in phase transfers could cause damage to the equipment.
Do to the variable nature of anomalies that do not consistently appear in a signal, designs for zero crossing detectors may not consistently be effective in controlling false zero crossings. For example, power sources used for various applications often supply transient type loads, and noise sources are not consistently turned on. Therefore, a noise transient may be inserted into a signal and cause a false zero crossing detection.